1. Field of the Invention
The invention relates to a jet engine comprising, from upstream to downstream (the upstream and downstream directions being defined by the direction of circulation of the primary flow), a high-pressure compressor, a diffuser grating and a combustion chamber, said high-pressure compressor comprising an external shell which radially delimits the duct for said primary flow and is connected to an annular structure extending radially outward, said diffuser grating comprising in the axial continuation of said external compressor shell an external casing connected to a rearwardly oriented conical strut delimiting, upstream, the end of said combustion chamber, said strut itself being connected to an external casing shell which extends in the upstream direction and is fastened to said annular structure by fastening means, said strut, said external casing shell and said annular structure defining a cavity around said diffuser grating, air bleed orifices being made in said strut in order to bring the end of the combustion chamber into communication with said cavity, and said external casing shell being equipped with outlet vents for the bled air.
2. Discussion of the Background
Air required for the cabin of the airplane equipped with at least one jet engine is bled off at the end of the combustion chamber in a region where it has the least disruptive effect on the overall efficiency of the engine. Bleeding takes place through the orifices in the strut, which makes it easy to install the outlet vents for the bled air. This arrangement requires relative sealing between the duct of the high-pressure compressor and the cavity situated above the grating of the diffuser.
This sealing is all the more difficult to achieve because the relative displacements between the diffuser grating and the external shell of the compressor are of the order of 1.5 mm in the axial direction and substantially of the same order in the radial direction, owing to the thermal and mechanical responses of the various components in an environment subjected to high pressures which may reach 30 bar and to high temperatures which may reach 650° C.
The current technology adopted to provide sealing between the compressor and the external casing of the grating is of the type comprising a seal made up of a strip and counterseal with springs pressing against these. This technology in fact allows a sufficiently large displacement between the two components.
The prior art is illustrated by FIG. 1, which shows the last stage of a high-pressure compressor 1 of a jet engine having, from upstream to downstream in the direction of the primary flow Fl, a ring of fixed vanes 2 extending radially inward from an external casing 3, followed by a ring of moving blades 4 mounted at the periphery of a compressor wheel 5 and extending outward as far as an external compressor shell 6 which, together with the external casing 3, radially delimits the duct for the primary flow, this external shell 6 being connected to an annular structure 7 which has a V-shaped cross section in the plane containing the axis of the jet engine and extending radially outward and is fastened to the external casing of the engine by bolting.
Provided downstream of the compressor 1 is a diffuser grating 10 which receives the compressed air from the compressor 1 and delivers it toward a combustion chamber 11. In the axial continuation of the external shell 6 of the compressor 1, the grating 10 has an external casing 12 connected to a conical strut 13 oriented toward the rear of the jet engine, this strut 13 defining the upstream wall of the end of the combustion chamber 11 and being connected in its radially outer region to an external casing shell 14 which extends in the upstream direction and has an upstream flange 15 by means of which the assembly consisting of the combustion chamber and the diffuser can be fastened on a radially outer flange 16 of the annular structure 7 by bolting.
A cavity 20 surrounding the diffuser grating 10 is thus delimited axially by the annular structure 7 and the strut 13, radially outwardly by the external casing shell 14 and radially inwardly by the downstream portion 6a of the external compressor shell 6 and by the upstream portion 12a of the external casing 12, a gap 21 separating these two portions.
The strut 13 has air bleed orifices 22 at the end of the combustion chamber and the external casing shell 14 is equipped with outlet vents 23 to supply a flow of air for aerating the cabin of the airplane or for cooling other elements of the jet engine.
Sealing between the compressor duct and the cavity 20 is achieved, as is shown in detail in FIG. 2, by a sectorized seal made up of strips 30 lined with counterseals 31, this seal being mounted on the periphery of the upstream portion 12a of the external casing 12 of the diffuser grating. To this end, this upstream portion 12a has over its periphery a channel 32 delimited by two flanges, the upstream one having the reference 33a and the downstream one having the reference 33b, which flanges have holes drilled into them for fastening rivets 34. The strips 30 and the counterseals 31 are kept in bearing contact with the downstream face of the upstream flange 33a by means of springs 35 and are retained by the rivets 34. The springs 35 are likewise retained by the rivets 34. The radially internal portion of the annular structure 7 has an annular projection 40 which extends axially into the cavity 20 and the end of which is situated above the upstream flange 33a in the absence of any axial displacement between the external shell 6 of the compressor 1 and the external casing 12 of the diffuser, as is shown in FIG. 2.
The springs 35 bear on the seals in the annular region separating the projection 40 from the upstream flange 33a. Moreover, the air pressure in the cavity 20 is slightly greater than the pressure in the duct at the gap 21.
The bearing points for the seals 30 on the projection 40 side and on the upstream flange 33a side have convex surfaces. The combined forces of the springs 35 and the pressure difference across the two faces of the seals 30 press the strips 30, which are flat, against these surfaces in the configuration shown in FIG. 2, thus providing sealing.
In certain flight phases, the bearing between the strips 30 and the projection 40 leaves an escape clearance, especially when the projection 40 passes above the channel 32, as is shown in FIGS. 4 and 5. Between two consecutive springs, the strips 30 move away from the projection and only the pressure difference between the two faces, which is small, may prevent the creation of this separation. An escape clearance 41 is then formed between the strips and the end of the projection 40.
When, by contrast, the diffuser grating 10 moves away from the compressor 1, as can be seen in FIG. 3, the force due to the pressure difference and the force of the springs 35 allow correct sealing to be achieved, by deformation of the strips 30.
The double arrows shown in FIG. 2 indicate the relative axial and radial displacements between the downstream end of the external compressor shell 6 and the upstream end of the external casing 12 of the diffuser grating 10.
It should also be noted that the arrangement of this sealing system borne by the external casing 12 makes it possible for the combustion chamber/diffuser assembly to be assembled on the compressor by relative axial displacement of said assembly with respect to the compressor and then by bolting together the external flanges 15 and 16.